One measuring method, out of a number of measuring methods for ascertaining fill level in a container, is the travel time measuring method. With this measuring method, for example, microwaves are radiated via an antenna device and the waves reflected from the surface of the medium are detected, with the travel time of the measuring signal being a measure of distance. From half the travel time, the fill level of the medium in a container can, in this way, be ascertained. The echo curve represents, here, the entire curve of the signal as a function of time, with each measured value of the echo curve corresponding to an amplitude of an echo signal reflected at a surface at a certain distance. The travel time measuring method is essentially divided into two evaluation methods: in the time difference measurement method, the time, which a broadband wave, signal pulse requires for a traveled distance, is determined, and in the frequency modulated, continuous wave method (FMCW—Frequency Modulated Continuous Wave), the transmitted, frequency-modulated, high frequency signal is compared with the reflected, received, frequency-modulated, high frequency signal. In the following, no restriction is made to any particular method of measurement.
In the case of certain process applications, fill level measuring devices are exposed to extreme conditions, for example high temperatures, high pressures and/or chemically aggressive substances. In particular, microwave, fill level measuring devices contain temperature, and/or pressure, sensitive components. These include, for example, measuring device electronics and transmission and/or reception elements for the microwaves.
Insertion of a hermetically sealed, process isolation element into the hollow conductor of the antenna ensures highest possible safety, since a second “safety element” seals the process, during an isolating of the modular, measurement active parts, such as e.g. a coupling element/exciter element or the measuring device electronics, from the measurement passive parts, such as e.g. the antenna, for maintenance or repair.
This problematic and a solution therefor are considered in European Patent EP 0 943 902 A1. There, a fill level measuring device working with microwaves is described for high temperature applications. The device has an antenna and includes a process isolation element in the hollow conductor region of the antenna. A glass window, among others, is described as a process isolation element. These glass windows protect the sensitive components of the fill level measuring devices against extreme measurement conditions, such as high temperatures, high pressures, and chemically aggressive media. A disadvantage of this design of the process isolation element is that the glass window must, because of the available production technology, for example due to the different material expansions, be provided in a thin-walled metal sleeve. This sleeve with the glass window must be soldered or welded in further, complicated, working steps into the hollow conductor. This requires a high additional work effort associated with the production of the antenna of the fill level measuring device. In addition, with the many working steps, manufacturing costs and safety risk are increased due to manufacturing errors.
Published U.S. application, US 2005/0253751 A1, describes a modular construction of a horn antenna. The process isolation element is constructed in the form of a ceramic, matching cone that is introduced into the hollow conductor and sealed by graphite packing rings. This design has the disadvantage that sealing against gas diffusion and a temperature resistant, process isolation are not achieved.
In German patent, DE 199 50 429 A1, a ceramic process isolation element is described that is shrunk fit into the hollow conductor. Disadvantageous, here, is that, despite polished bounding surfaces on the process isolation element and in the waveguide, no seal is achieved. Further, the large compressive forces that act on the ceramic, process isolation element can lead to stress cracks.
A disadvantage of the aforementioned examples of embodiments of a state of the art process isolation element is that manufacture is very complex and expensive. In order to obtain a connection impervious to gas diffusion between a ceramic and a surrounding metal, hollow conductor, only a soldering procedure is well-known according to the state of the art. In such case, the ceramic, as process the isolation element, is first metallized on the surface in complex working steps, then soldered into a soldering sleeve, which has a coefficient of thermal expansion similar to that of the ceramic (e.g. Kovar), and this finally is welded into a stainless steel, hollow conductor. Other joining techniques, such as, for example, shrink fitting at high temperature, always have a certain leakage rate and are not impervious to gas diffusion, as already mentioned.